4/1/03 1 Spring 2003 CS118 Computer Network Fundamentals  Instructor: Lixia Zhang  Office: 4531G Boelter Hall  Office hours:  Tuesdays: 4:00-5:00pm  Thursdays: 1:00-2:00pm  Other times: appointment by  When me: pls put cs118 in the subject line  TAs:  Vasileios Pappas  Jon Canan  Course homepage:

4/1/03 2 What this course is about  What are the underlying concepts and technologies that make the Internet run?  First/introductory course in computer networking  Learn basic networking technologies & principles  Develop network programming skills

4/1/03 3 Course Workload  Reading assignment for every lecture  Weekly homework assignment  Assigned every Thursday (except the 10th week)  Due 6:00pm the following Thursday to TA's mailbox; homework solutions posted Friday morning.  Two programming projects  Check class website for details  Midterm and final exams  Last but not least: Classroom participation

4/1/03 4 Grading breakdown  Homework: 20%  Projects: 30%  Midterm: 20%  Thursday, May 8th  Final exam: 30%  3:00-6:00PM, Saturday June 7th

4/1/03 5 Course Policies  no late turn-in is accepted for credit  no make-up exams  no misconduct

4/1/03 6 Lets have a shared understanding  Why are you here?  The goal in next 10 weeks  What I can help  Whose fault would it be, if you:  failed to understand the lecture material  failed to turn in an assignment on time  fell to sleep in class  Missed one of the exams

4/1/03 7 Lecture Teaching  "Taking notes in class helps me understand better"  So posted lecture notes will be like this Web servers Web client Slide title Client communicates with the server through the net (your notes go here)

4/1/03 8 Part I: Introduction Today's goal:  get context, overview, “feel” of networking  more depth, detail later in course Overview:  what’s the Internet  network edge, network core, access net, physical media  Circuit switching vs. packet switching  Protocols, protocol layers  Performance measure: data loss, delay Assignment: Read chapter 1

4/1/03 9 What’s the Internet: “nuts and bolts” view  hosts (end-systems):  pc’s workstations, servers, PDA’s phones, toasters..  Send/receive data, but do not forward  Connected to networks made of  communication links  fiber, copper, radio, satellite  routers: forward chunks of data (packets) through a network  running network applications  WWW, , games, e- commerce, file sharing (MP3) local ISP Campus network regional ISP router workstation server mobile

4/1/03 10 A closer look at network structure:  network edge: end systems (hosts)  run application programs at “edge of network”  client/server model

4/1/03 11 A closer look at network structure:  network edge: end systems (hosts)  run application programs at “edge of network”  client/server model  access networks  Physical media, communication links

4/1/03 12 Access Networks  Compus: local area network (LAN)  Ethernet:10 Mbps, 100Mbps, Gigabit Ethernet  wireless LANs:  radio spectrum replaces wire  wider-area wireless access: CDPD  wireless access to Internet via cellular network  Dialup via modem, ADSL (asymmetric digital subscriber line) base station mobile hosts router

4/1/03 13 A closer look at network structure:  network edge: end systems (hosts)  run application programs at “edge of network”  client/server model  access networks  Physical media, communication links  network core:  mesh of interconnected routers  network of networks  the fundamental question: how is data transferred through the net?  Circuit switching  Packet switching

4/1/03 14 How multiple data transfers share the same network Circuit Switching vs. Packet Switching  Circuit switching  dedicate link bandwidth & switch capacity to each “call”  Requires call setup  Guaranteed performance  Packet switching  Packet: small chunks of data  Send packets as soon as link available  switch receives a full packet then forwards it towards the destination

4/1/03 15 Circuit Switching: FDM and TDM FDM frequency time TDM frequency time 2 users Example:

4/1/03 16 Packet Switching: Statistical Multiplexing  Store-and-forward  Packet switch can temporarily buffer up packets  Introduce queueing delay  Packets get dropped when the queue is full

4/1/03 17 Packet switching versus circuit switching  each user:  100,000 bits/sec when “active”  active 10% of time  circuit-switching:  10 users  packet switching:  35 users: Prob.(n > 10)  Mbps link A number of issues related to packet switching:  How does a router figure out where to forward packets?  What if packets get lost? Or get garbled along the way? Example: how many users can share a 1 megabits/sec (1 Mbps) link?

4/1/03 18 Tentative Course Schedule:  Introduction (2 lectures, textbook: Chapter 1)  Network applications (3 lectures, Chapter 2)  Socket programming (1 lecture)  Transport protocols (3~4 lectures, Chapter 3) Midterm exam (in class)  Network protocols (4 lectures, Chapter 4)  Link Layer: LANs (3 lectures, Chapter 5)  Network Security (1 lecture, Chapter 7) Review for final exam

4/1/03 19 Or more intuitively my computer server Link layer (week 8-9) network layer (week 6-7) Transport (week 4-5) Applications (week 2-3) Week 1

4/1/03 20 What to do after this class  Take a look at the course homepage  Finish reading assignment before next lecture  Food for thought:  What makes the Internet so popular these days?  What's lay ahead?  Interesting articles on Internet histories  "Some Perspectives on Networks  Past, Present and Future" by Paul Baran, 

4/1/03 21 Host Application Host Web Host video Packet Switched Networks Host Network protocols govern all communication activities in a network

4/1/03 22 Hello Hi Got the time? 2:00pm What’s a protocol? conn. reply Connect. req. Get

4/1/03 23 Dave's computer Jim's computer One example: send

4/1/03 24 Organization of air travel  a series of steps ticket (purchase) baggage (check) gates (load) runway takeoff airplane routing ticket (complain) baggage (claim) gates (unload) runway landing airplane routing

4/1/03 25 Distributed implementation of layer functionality ticket (purchase) baggage (check) gates (load) runway takeoff airplane routing ticket (complain) baggage (claim) gates (unload) runway landing airplane routing Departing airport arriving airport intermediate air traffic sites airplane routing

4/1/03 26 Internet protocol stack application transport network link physical

4/1/03 27 Dave's computer Jim's computer One example: send application transport network link physical application transport network link physical application transport network link physical network link physical data

4/1/03 28 application transport network link physical application transport network link physical source destination data H t H t H n H t H n H l H t H t H n H t H n H l message segment datagram frame Protocol layering and data

4/1/03 29 Protocol header: one examples destination address source address type data … error checking code Link layer: Ethernet frame format header trailer application transport network link physical

4/1/03 30 Data Delivery Performance  3 basic measuremetns my computer server End-to-end Hop-by-hop

4/1/03 31 Packet Losses  Loss due to congestion  Loss due to transmission errors  wireless links

4/1/03 32 Throughput  Throughput over a single link  Point-to-point  Multi-access  Throughput between two end hosts

4/1/03 33 Delay in packet-switched networks 4 sources of delay at each hop  nodal processing:  Queueing A B propagation transmission nodal processing queueing C  Transmission  Propagation = L / R = d/s

4/1/03 34 total delay (A  B) = ?  Queuing delay =  transmission delay =  Propagation delay = Switch A Switch B link length = 100 km Bandwidth= 1 Mbps packet size= 1000 bits (all pkts equal length) (2.0x10^8 meters/sec in a fiber) Example: one hop delay

4/1/03 35  Time to send a packet from point A to point B  sum of delays across each hop along the path  RTT: round-trip-time Network latency A B

4/1/03 36 Packet-switching: store-and-forward  Takes L/R seconds to transmit (push out) packet of L bits on to link of R bps  Entire packet must arrive at router before it can be transmitted on next link: store and forward  Ignore propagation delay R R R L

4/1/03 37 Packet-switching: store-and-forward Example 2:  A sends 5 packets to B  L = 8000 bits, R = 2 Mbps  Ignore propagation delay  How long does it take starting from A sending the first bit of first packet till B receives the last bit of the last packet? R R R time T=0 1 L/R 2L/R 1 3L/R L/R L/R L/R L/R

4/1/03 38 Bandwidth, transmission delay, and propagation delay bandwidth time 10Mbps 20Mbps Propagation delay= 10ms Bandwidth= 1 Gbps (bandwidth  delay) product: amount of data "in-the-pipe" 125KB data in the pipe